Comparison and QSAR study of thermodynamic pKBH+ values of benzoylhydrazones determined by UV-Vis spectroscopy

Abstract

Thermodynamic dissociation constants of fifteen protonated aromatic hydrazones were determined by UV-Vis spectroscopic method, as one of the main methods for determination of pK_BH⁺ values of a weak acids and bases. The stoichiometric pK_BH⁺ values of investigated compounds were determined at three different ionic strengths (0.1, 0.25 and 0.5 mol/dm³) in acidic media (1 < pH < 7). The thermodynamic pK_BH⁺ values were evaluated graphically as an intercept with extrapolation to zero ionic strength of the curve which represents dependence of ionization constants on square root of ionic strength. The obtained results showed that there are no significant differences in the values of thermodynamic dissociation constants of investigated hydrazones with no substituent and metoxy group (–OCH₃) on hydrazide part of the molecule. The differences in the pK_BH⁺ values were noticed for hydrazones with nitro group (-NO₂) on the benzene ring of benzaldehyde. Nitro hydrazones have lower pK_BH⁺ values, probably due to the influence of the -NO₂ group present in their molecule. The obtained pK_BH⁺ values were in accordance with those for similar class of compounds. QSAR study has been made by correlation thermodynamic ionization constants with structure using various physicochemical parameters.